1. Field of the Invention
The invention relates generally to the field of seismic surveying of subsurface rock formations. More specifically, the invention relates to seismic energy sources that can be used in wellbores drilled through rock formations.
2. Background Art
Seismic geological mapping techniques known in the art include seismic surveys made from within wellbores drilled through the earth. Such surveys are known in the art as “vertical seismic profile” surveys (“VSPs”). One objective of obtaining a VSP in a wellbore is to be able to determine the velocity of seismic energy through the various layers of the earth by directly measuring seismic travel time from the earth's surface to a known depth within the wellbore in the earth. Velocity information is important in order to infer depths of subsurface structures mapped from surface-acquired seismic surveys.
One type of VSP technique includes actuating a seismic energy source at the earth's surface and measuring seismic travel time to a seismic receiver disposed in the wellbore at known depths. Typically such receivers are lowered to selected depths in the wellbore at one end of an armored electrical cable (“wireline”). Another VSP method, called “inverse VSP”, includes positioning a seismic energy source in a wellbore at selected depths, actuating the source and detecting seismic energy using receivers disposed at the earth's surface. Various types of seismic sources and seismic receivers are known in the art for use in a wellbore.
Although the foregoing VSP techniques are referred to for convenience as “wireline” techniques, the discussion below with respect to the limitations of such techniques is equally applicable to VSP techniques where the source or receiver is conveyed into the wellbore by means of drill pipe, coiled tubing, or the like. Irrespective of the actual conveyance mechanism used, wireline VSP techniques known in the art typically require that the wellbore already be drilled in order to position the source or receiver at any selected depth in the wellbore. In many instances, it is desirable to have an estimate of seismic velocity prior to actually drilling through particular formations, not the least important reason for which is because some formations have fluid pressure in pore spaces therein which exceeds pressures normally encountered at identical depth levels. As is well known in the art, estimates of seismic velocity may be used to make estimates of fluid pore pressure prior to drilling through these formations. Estimates of such pressures may be made, for example, using VSP techniques known in the art by temporarily stopping drilling, and inserting a receiver or source into the wellbore at or near the bottom of the wellbore and taking a so-called “checkshot” survey. In a checkshot survey, a seismic travel time from the known depth in the wellbore to the earth's surface is used to “calibrate” seismic surveys made entirely at the earth's surface in order to better estimate formation fluid pressure in as-yet-undrilled formations. However, stopping drilling to make checkshot surveys using techniques known in the art is time consuming, and thus expensive.
It is known in the art to include a seismic receiver in the drill string (drilling tool assembly) during drilling operations in order to reduce the time used to obtain VSP data while a wellbore is being drilled. In this technique, a seismic source is actuated at the earth's surface, as in other types of VSP surveys, and signals are recorded in appropriate circuits coupled to the receiver in the wellbore. Several types of wellbore seismic receivers for use during drilling are known in the art. See for example, U.S. Pat. No. 5,555,220 issued to Minto. A limitation to the technique of obtaining a VSP survey while drilling using a receiver in the drill string is that the broad range of signals detected by the receiver typically cannot be completely interpreted with available downhole processing means until the drill string (having the receiver therein) is removed from the wellbore. It is necessary to remove the receiver from the wellbore and interrogate the contents of the recording device because while-drilling measurement systems known in the art are typically limited to relatively slow forms of signal telemetry, such as mud pressure modulation or low-frequency electromagnetic telemetry. While-drilling telemetry systems known in the art are generally limited to a data rate of about 5 to 10 bits per second. As a result, even with data compression techniques known in the art, interrogating a wellbore seismic receiver substantially in real time is impracticable. Another operating consideration when using drill string mounted seismic receivers is the need to substantially stop drilling operations during the times at which seismic signals are to be detected. In many cases, the amount of acoustic noise caused by movement of the drill string within the wellbore is such that detecting seismic signals is difficult while drilling operations are in progress.
Another while-drilling VSP survey technique known in the art uses the drill bit as a seismic energy source. In this technique, a pilot sensor is mounted at the top of the drill string, and seismic sensors are deployed at the earth's surface. Signals detected by the seismic sensors are cross-correlated to the signals detected by the pilot sensor to determine the impulse response of the earth. Drill bit VSP techniques known in the art include methods for determining a closer representation of the drill bit seismic signature, and time correcting the pilot signal for seismic travel time through the drill string. Limitations of drill bit-source VSP techniques known in the art include, foremost, that roller cone drill bits must be used. In many drilling situations, it is preferable to use fixed cutter bits, such as polycrystalline diamond compact (“PDC”) bits. In such cases, it has proven substantially impossible to obtain a usable seismic signal from the bit. It is also known in the art that the seismic energy radiation pattern of roller cone bits is such that when the wellbore inclination from vertical exceeds about 30 to 40 degrees, the amount of seismic energy reaching the earth's surface proximate the equivalent surface location of the wellbore is very small. As a result of the limitations of bit-source VSP methods known in the art, the practical applications of bit-source VSPs have been limited.
Alternatively, a seismic energy source can be positioned in the wellbore and actuated at selected times during drilling. Seismic sources known in the art for use while drilling have generally not performed sufficiently well to be commercially useful. As a result, there is a need for an improved seismic energy source for use while drilling operations are in progress.
Another seismic energy source for use while drilling a wellbore is disclosed in U.S. Patent Application Publication No. 2004/0240320 filed by McDonald et al. The source according to this aspect of the invention includes a drive shaft adapted to be coupled in a drill string, and a housing rotatably supported outside the drive shaft. At least one contact member is disposed on an exterior of the housing and is selectively urged into contact with a wall of a wellbore surrounding the housing. The source includes means for selectively controlling a force applied to the at least one contact member.
Another type of seismic energy source usable within wellbores is described in U.S. Patent Application Publication No. 2009/0154290 filed by Radtke et al., the underlying patent application for which is commonly owned with the present invention. The source described in the foregoing publication includes a method for operating an impulsive type seismic energy source in a firing sequence having at least two actuations for each seismic impulse to be generated by the source. The actuations have a time delay between them related to a selected energy frequency peak of the source output. One example of the method is used for generating seismic signals in a wellbore and includes discharging electric current through a spark gap disposed in the wellbore in at least one firing sequence. The sequence includes at least two actuations of the spark gap separated by an amount of time selected to cause acoustic energy resulting from the actuations to have peak amplitude at a selected frequency. While a single pulse creates frequencies which increase with depth, the frequencies generated by the method described by Radtke et al. can be independent of depth by suitable operation of the sparker. However, for frequencies greater than 40 Hz, the power supply must be capable of generating very rapid pulse rates, or using multiple sparkers and power supplies pulsing at slower rates and timed to produce the desired fundamental frequency. It is desirable to have a system that does not require a special power supply is described herein.
A limitation to using a conventional sparker is that the frequency of the seismic energy emitted increases as the vertical depth of the source in the wellbore increases. The depth will generally be related to the hydrostatic pressure in the wellbore and thus on the source. The reason for the foregoing can be explained as follows.
When an underwater high energy spark impulse occurs, a bubble is formed that expands outward until the pressure inside the bubble reaches ambient pressure and then the bubble collapses. This process produces two high energy pressure pulses, one at the initial impulse and one on bubble collapse. The time between these two pressure pulses is referred to as the bubble period. The bubble period is a function of the energy involved in the initial impulse and the ambient pressure. Theoretical calculations of the bubble period are based on the modified Rayleigh-Willis formula shown below.T=(0.000209)(0.7)(KQ)1/3/(d+33)5/6 
Wherein T is the bubble period in seconds, d is the depth in feet, K is a constant (1010) when Q, the energy is measured in kilojoules
The frequency spectrum produced by this impulse is a mean frequency that is approximately the reciprocal of the time difference between the formation and collapse of the bubble.
What is needed is a seismic energy source for use in a drill string that has selectable frequency output independent of depth of the source in the wellbore.